PROJECT SUMMARY/ABSTRACT Foxp3+CD4+ regulatory T (Treg) cells control most types of immune responses, influencing the activities of both innate and adaptive cell-types. Evidence is mounting that special classes of Tregs also regulate non- immunological processes in non-immunological cells ? and, as a consequence, organismal homeostasis. The best characterized example to date is a Foxp3+CD4+ population that resides in visceral adipose tissue and regulates local and systemic metabolic indices. Recently, a distinct Treg population was identified in murine skeletal muscle. Muscle Tregs are highly enriched within the local CD4+ T cell compartment, and have a distinctive T cell receptor repertoire and transcriptome. They regulate muscle regeneration in response to both acute and chronic injury, affecting the behavior of infiltrating innate cells as well as impacting muscle progenitor cells, at least in part via secretion of the growth/survival factor, Amphiregulin. An analogous population of Foxp3+CD4+ T cells has been identified in humans, enriched in patients with Duchenne muscular dystrophy. Preliminary data provide strong evidence that IL-33 controls Treg accumulation in injured skeletal muscle. For example, this cytokine is deficient in muscles of geriatric mice, as are Tregs, partially explaining the poor muscle repair characteristic of old animals; and this cytokine?s experimental supplementation coincident with injury significantly improves muscle repair/regeneration. Il-33 has an intriguing pattern of expression in muscle: primarily in fibro/adipogenic progenitor cells, often (though not always) in close association with archetypal nerve structures, such as nerve bundles and muscle spindles. The major goal of this proposed project is to elucidate the cellular and molecular interactions between Tregs and IL-33-producing cells during acute and chronic muscle injury. This goal will be addressed in the framework of three Specific Aims: 1) to paint a more precise picture of IL-33-producing cells in skeletal muscle; 2) to evaluate the functional relevance of the physical association between IL-33-producers and nerve structures in muscle, and 3) to explore the role of IL- 33 in a chronic muscle disease. Successful completion of these Aims stands to advance our understanding, and potential treatment, of several muscle pathologies ? including the reduced muscle mass, function and repair of the aged, muscular dystrophies such as Duchenne muscular dystrophy, and potentially Amyotrophic Lateral Sclerosis.